The present invention relates to optical systems for inspecting multi-layer silicon devices, and more particularly to an optical system employing near infrared illumination to detect defects in silicon layers.
Inspection systems are available in the art for detecting defects such as cracks in silicon layers, particularly in silicon layers or wafers employed in solar cells. Such systems include a source of linearly polarized infrared illumination which is directed entirely onto the solar cell device at a first selected angle. The linearly polarized infrared illumination is reflected from the silicon layer of the solar cell and an infrared video camera is positioned at a second selected angle with respect to the solar cell. The video camera, which includes an infrared linear polarization analyzer, provides a visual image of the silicon layer and cracks in the silicon wafer surface can be observed.
A typical infrared solar cell inspection system is described in the publication by J.R. Hodor, H.J. Decker, Jr., J. Barney, "Infrared Technology Comes to State-of-the-Art Solar Array Production" SPIE Vol. 819, Infrared Technology XIII (1987), pp. 22-29.
The systems previously known in the art employ infrared illumination in the 5 to 15 micron range, and also aim the collimated, linearly polarized infrared illumination directly onto the solar cell. Such systems result in an image on the video camera screen in which cracks in the silicon layer can be observed.
Available inspection methods, known in the art are extremely complex in design and use, and hindered by large weight constraints. All of the various systems known or proposed to date may no longer be considered practical since the complexity, size and weight constraints of the systems make them too expensive and not portable enough for use in a dynamic aerospace manufacturing and test environment.